In contrast to traditional point-like inorganic light emitting diodes (LEDs) that are based on complex crystalline structures, OLEDs are flat light sources that use organic semiconductors to generate light. They consist of a (glass) substrate, a transparent electrode, one or more thin organic layers, and a counter-electrode which can also be transparent. The component is encapsulated to protect against oxidation and moisture.
“An OLED operates on the principle of injection electro-luminescence, just like an inorganic LED. Positive and negative charge carriers are made to give off light by recombining in an emission layer. In other words, light is emitted in a semiconductor – an inorganic semiconductor in the case of an LED and an organic one in the case of an OLED. The structure of the molecules determines the colour of the light; in the case of LEDs the light colour is determined by the crystal structure of the semiconductor materials.”
At present, OLEDs with a typical luminous efficacy of 25lm/W are more efficient than conventional incandescent lamps or halogen lamps but have not yet achieved the efficiency of fluorescent lamps or LEDs. Through OSRAM’s OPAL research project, however, laboratory samples have already achieved an efficiency of 62lm/W, putting them on a par with fluorescent lamps.
“OLEDs in white are typically achieving an efficiency of 25lm/W and a life of more than 5,000 hours (drop in brightness to 70% of the initial luminance of 1,000cd/m²). These figures mean that they are already better than conventional halogen lamps.
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“Their pleasant warm white light colour is similar to that of a classic light bulb (colour coordinates CIE x=0.46/y=0.42). With a colour rendering index (CRI) greater than 80, OLEDs offer high colour quality. They cover a broad wavelength spectrum so they provide a natural colour impression.”
New OLED architectures are being developed in current R&D projects which will take efficiency levels up to 100lm/W and will then be available in two or three years time.
In contrast to inorganic LEDs, OLEDs are particularly efficient in the warm-white emission range. LED development is further advanced in certain parameters such as lifespan and temperature stability. But, OLEDs are catching up fast in terms of efficiency, colour temperature and colour rendering.
“OLEDs are more than just light sources, they are design elements. Even when they are switched off they look very different from conventional light sources. They are very thin, very flat, very lightweight and, therefore, very attractive. They can also be transparent, diffused or mirrored so they offer a completely new look.
“Plus, they can be easily dimmed to any level by changing the operating current. In contrast to tubular and compact fluorescent lamps they emit their full output as such as they are switched on.”
At present, the costs for light from OLEDs are still considerably higher than the market prices for conventional light sources. Industrialisation of the new substrate and encapsulation processes will lead to substantial reductions in costs.
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